High permeability edema is the hallmark of acute lung injury (ALl). Mechanical ventilation, the mainstay of supportive therapy for ALl, may exacerbate the injury by disrupting the epithelial and endothelial barriers contained within the alveolar septae through repetitive stress. Sphingosine-l-phosphate (Sph-l-P), a biologically active phospholipid, has been demonstrated to enhance endothelial layer barrier function in vitro. The overall goal of this proposal is to establish Sph-l-P as a novel therapy for the prevention of ventilator-associated lung injury (VALl), and to investigate the cellular mechanisms by which it confers protection in vivo. The pathophysiologic and cellular mechanisms by which mechanical ventilation exacerbates LPS-induced lung injury on both global and regional levels in a canine model will be explored. In doing so, we will quantify shunt fraction, alveolar dead space fraction, lung compliance, lung edema formation, and alveolar barrier permeability. Regional alveolar injury and lung mechanics will be quantified by specific CT imaging techniques (SA#1). Subsequently, a series of experiments designed to evaluate the effect of exogenous Sph-l-P on the physiologic and mechanical parameters describing lung injury will be conducted in this model (SA#2). In conclusion, functional genomic analyses will be employed to elucidate regional differences in gene expression throughout the heterogeneously injured lung and regional cellular alterations induced by the novel therapeutic agent Sph-l-P (SA#3).